Project Summary/Abstract Prostate cancer arises as an androgen driven disease and therefore therapies targeting the androgen receptor (AR) have been a major focus of prostate cancer treatment. Despite recent advances in the development of highly effective AR-directed therapies, the development of acquired resistance remains a significant challenge. An emerging concept of anti-AR resistance is the induction of epithelial plasticity to a heterogeneous state that has lost its AR-dependent luminal identity and ultimately develops neuroendocrine prostate cancer (NEPC). There are no effective therapies for patients with NEPC and prognosis is extremely poor (average survival = 7 months). NEPC retains many of the genomic alterations that arise in prostate adenocarcinoma castration resistant prostate adenocarcinoma suggesting a clonal origin. Recently, we and others have identified and validated new therapeutic targets and drivers of cell transformation from CRPC to NEPC (e.g. induction of MYCN (encodes N-Myc) or loss of Retinoblastoma-1 (RB1) and TP53. We have shown that N-Myc is over- expressed in the majority (>95%) of NEPC cases and in 20% of CRPC tumors that also display features of NEPC. RB1 loss occurs in majority of NEPC cases (70%) and in 32% of CRPC tumors which overlaps, in part, with N-Myc over-expression. Currently, the synergy between MYCN induction and RB1 loss, mechanisms downstream of induction of MYCN induction/PTEN loss with or without RB1 loss in driving lineage switching and treatment response are not well understood. Our over-arching hypothesis is that specific molecular alterations (e.g. MYCN induction) in prostate cancer cells drive lineage plasticity by establishing a molecular program associated with the neural lineage and epigenomic reprogramming as a mechanism of resistance to anti-AR therapy and transformation towards a neuroendocrine phenotype. To address this hypothesis we have formulated the following three Specific Aims: We will use murine and human in vitro, in vivo and ex vivo models to establish the role of N-Myc and downstream mediators (e.g. NKX2-1, SOX11,) in gene expression and epigenetic reprogramming driving CRPC-Adeno towards NEPC (Aim 1); we will also define essential N-Myc- transcriptional complex proteins that mediate the transition from CRPC-Adeno towards NEPC (Aim 2); finally, we will evaluate the preclinical efficacy of therapy targeting CRPC-Adeno to NEPC transition (Aim 3). We expect that during the transformation process and before epigenetic hardwiring, tumor cells will retain the capacity to revert to a luminal phenotype as a result of molecular or pharmacological intervention. This project leverages unique model systems to study drivers of lineage switching and treatment response. The multidisciplinary project builds upon a long-standing collaboration between the PI and co-Is and extensive preliminary data. At the conclusion of this study, we will have a better understanding of the mechanisms underlying lineage plasticity associated AR-directed treatment resistance. !